1. Field of the Invention
The present invention relates to a reproduction apparatus constructed so as to be capable of reproducing information recorded on information recording media including at least a super RENS (super REsolution Near-field Structure) recording medium, and to an information recording medium testing apparatus that includes the reproduction apparatus.
2. Description of the Related Art
In recent years, commercialized DVDs (Digital Versatile Discs) are constructed so that marks with a diameter of just 0.40 μm or thereabouts formed on an information recording medium (an optical recording medium) can be distinguished and reproduced by using an optical head where the wavelength of laser light is in a range of 635 nm to 675 nm, inclusive and the numerical aperture of the objective lens is in a range of 0.59 to 0.66, inclusive to focus the laser light to a spot diameter of just 1 μm or thereabouts. In addition, BD (Blu-ray Discs) are constructed so that marks with a diameter in a range of just 0.138 to 0.160 μm, inclusive can be distinguished and reproduced by using an optical head where the wavelength of laser light is in a range of 390 nm to 420 nm, inclusive and the numerical aperture of the objective lens is in a range of 0.70 to 0.90, inclusive to focus the laser light to a spot diameter of just 0.48 μm or thereabouts, which makes it possible to record data at even higher density.
On the other hand, a so-called super RENS (super REsolution Near-field Structure) recording medium has been proposed in recent years, and research into such medium is presently being conducted. A super RENS recording medium has an optical signal amplifying effect (a “super resolution” effect) of around several tens in magnitude, with this effect making it possible to distinguish and reproduce marks of 0.1 μm and below (that is, marks that are smaller than the reproduction limit for optical reproduction) on a recording medium while using the same optical system as a DVD, a BD, or the like. Since the optical system used for a DVD, a BD, or the like can be used without modification, it is possible to use the construction of a standard optical disc apparatus such as that disclosed by Japanese Laid-Open Patent Publication No. 2002-319137, for example, to realize a reproduction apparatus for reading information recorded on a super RENS recording medium.
When using the circuit construction of the optical disc apparatus disclosed in the above Publication without modification in a reproduction apparatus for a super RENS recording medium, the present inventors discovered the following problem. For a super RENS recording medium, the super resolution effect is initially achieved when laser light with a reproduction power that is extremely high (for example, 2.6 mW to 4.0 mW inclusive for a DVD and 1.0 mW to 3.0 mW inclusive for a BD) compared to the reproduction power for a recording medium such as a DVD or a BD (0.7 mW for a DVD and 0.3 mW for a BW) is irradiated. This means that when the marks recorded on the recording medium are large marks with wide gaps in between the marks as shown by the region Win FIG. 3, the amplitude of an electric signal S1 outputted from an optical head that has received the laser light reflected by the recording medium becomes extremely large as shown in FIG. 3 compared to when laser light of a conventional reproduction power is irradiated. On the other hand, when the marks recorded on the recording medium are small marks with narrow gaps in between as shown in the region X in FIG. 3, the ratio of the DC component to the AC component in the electric signal S1 becomes extremely large, as shown in FIG. 3.
Here, an AGC (Auto Gain Control) circuit that is disposed in front of an A/D converter controls the gain for the electric signal S1 outputted from the optical head to control the amplitude of the electric signal S1 and thereby adjusts the amplitude to an optimal amplitude for the dynamic range of the A/D converter. However, since laser light with an extremely high reproduction power is irradiated onto a super RENS recording medium, as described above the amplitude of the electric signal S1 outputted from the optical head can become extremely large and the ratio of the DC component to the AC component can also become extremely large. For this reason, when the amplitude of the electric signal S1 exceeds the tolerated input limit of the AGC circuit, distortion occurs in the waveform of the electric signal S1 (the reproduction signal) outputted from the AGC circuit, while when the ratio of the DC component of the electric signal S1 is large, gain control by the AGC circuit becomes insufficient, resulting in increased jitter. When the electric signal S1 outputted from the AGC circuit becomes distorted or has increased jitter, it becomes difficult for the A/D converter to favorably carry out conversion to digital data, and it also becomes difficult for a demodulation circuit disposed after the A/D converter to correctly demodulate data based on such digital data. Accordingly, with such reproduction apparatus, there is the problem that it is difficult to correctly reproduce information recorded on a super RENS recording medium.